1. Field of the Invention
The present invention relates to a power crimp-type semiconductor device and, more particularly, to a crimp-type semiconductor device having a non-alloy structure using a crimp structure in, e.g., a reverse-blocking triode thyristor (SCR), a gate turnoff (GTO) thyristor, and a transistor in which a semiconductor pellet and an electrode member are not brazed to each other.
2. Description of the Related Art
In general, a structure of a power semiconductor device adopts a crimp package in order to satisfy special conditions such as heat radiation, a current capacity, blast proofing, and a diameter of a semiconductor pellet. Of these power semiconductor devices, FIG. 1 shows a prior art GTO thyristor as a crimp-type semiconductor device having a control electrode for controlling power. Note that in the accompanying drawings, members not bonded but in contact with each other are apparently separated for illustrative convenience.
FIG. 1 is a schematic sectional view of the prior art GTO thyristor. As shown in FIG. 1, a silicon semiconductor pellet 10 comprises a disk-like p-type emitter layer 12, n- and p-type base layers 14 and 16, and a plurality of n-type emitter layers 18. The n-type emitter layers 18 are a plurality of island regions formed on the p-type base layer 16, and an aluminium (Al) cathode electrode 20 is formed on each layer 18. An Al continuous gate electrode 22 is formed on a portion on the p-type base layer 16 at which no n-type emitter layer 18 is formed. An Al anode electrode 24 serving as a brazing material is formed on the lower surface of the p-type emitter layer 12. The circumferential surface of the semiconductor pellet 10 constituted by these layers is protected by an insulating silicon resin 26.
Cathode and anode electrode posts 28 and 30 each comprising copper are arranged to oppose each other at cathode and anode sides (upper and lower sides in FIG. 1), respectively, so as to sandwich the pellet 10. The cathode electrode post 28 is a partially hollow cylindrical member having a gate lead 32 (to be described later) therein. The cathode electrode post 28 crimps the cathode electrode 20 via a cathode electrode plate 34 and a thin cathode electrode plate 36 each comprising molybdenum (Mo), and the anode electrode post 30 crimps the anode electrode 22 via an anode electrode plate 38 comprising molybdenum (Mo). Note that the anode electrode 24 and the anode electrode plate 38 are brazed with each other.
A ceramic insulating cylindrical member 40 is arranged to surround the outer surface of the pellet 10. The cylindrical member 40 is silver-brazed to the cathode electrode post 28 via ring metal plates (kovar) 42 and 44 called a weld ring, and to the anode electrode post 30 via ring metal plates 46 and 48 of the same type, thereby constituting a package for air-tightly sealing its interior.
The gate lead 32 is arranged in the hollow and insertion portions of the cathode electrode post 28, the cathode electrode plate 34, and the thin cathode electrode plate 36 via an electrical insulating member (not shown). One end of the lead 32 is crimped to the gate electrode 22 by a gate crimp spring 50 via the electrical insulating member (not shown). The other end of the lead 32 is guided outside through a metal sleeve 52 brazed on the circumferential surface of the cylindrical member 40, and is sealed by a sealing portion 54.
When a forward voltage is applied on the GTO thyristor having the above arrangement to flow a gate trigger current through the gate electrode, the GTO thyristor is turned on. That is, a load current (ON current) is flowed from the anode electrode post 30 to the cathode electrode post 28 via the semiconductor pellet 10.
While a normal thyristor is turned off by flowing a main current in the opposite direction from a commutation circuit, the GTO thyristor is turned off by flowing a gate current in the opposite direction. For this reason, the n-type emitter layer of the GTO thyristor is divided into small islands so as to be easily turned off, and the gate electrode is formed to surround each island emitter layer.
A thermal expansion coefficient of the silicon pellet 10 largely differs from that of the electrode posts 28 and 30. Therefore, in order to protect the pellet 10 against a thermal stress, an electrode comprising tungsten (W) or Mo is formed between the pellet 10 and the posts 28 and 30. In particular, such an electrode plate having a proper thickness is brazed to the silicon pellet at the anode side.
In the silicon pellet backed with the electrode plate having high stiffness, almost no damage such as a crack is produced by a thermal stress of the electrode post and the like at the cathode side. In addition, even if the positions or sizes of the upper and lower electrodes differ, no large difference is produced in its characteristics.
The silicon pellet (to be referred to as a "silicon pellet having an alloy structure" hereinafter) brazed to an electrode plate consisting of W or Mo has the following problems.
(i) Although W or Mo having a thermal expansion coefficient close to that of silicon (Si) is used for an electrode plate, an influence caused by a difference between the thermal expansion coefficients cannot be completely eliminated, thereby producing warping in the pellet of an alloy structure. Therefore, since a crimp pressure becomes nonuniform, reliability is not improved.
(ii) Since a W or Mo plate is heavy, the pellet having an alloy structure is inconvenient to be carried in the manufacture. For example, it is difficult to manufacture the pellet having an alloy structure by using the same manufacturing line as a pellet process of an IC or the like.
(iii) Al as a brazing material and Si form an alloy which is bonded to a brazed portion between an electrode plate and a silicon pellet. In this case, a eutectic alloy of Al and Si is sometimes produced and partially projects in an Si substrate, thereby degrading a withstand voltage or characteristics of an element.
(iv) Since an Al projection or a thick brazing layer (&gt;20 .mu.m) is formed at the brazed portion, a shallow diffusion layer cannot be formed on a pellet at a brazing side (anode side).
In order to solve the above problems, a device having a structure (to be referred to as a "silicon pellet having a non-alloy structure" hereinafter) in which an anode electrode of a silicon pellet and an electrode plate are not brazed with each other has been proposed.
Since, however, a single silicon pellet not brazed to an electrode plate is brittle, the silicon pellet may crack due to a difference in thermal expansion coefficient between the silicon pellet and the electrode plate when it is crimped or vibrated or subjected to a temperature cycle test or a thermal fatigue test.
FIG. 2 is a partial plan view schematically showing a positional relationship between patterns (indicated by solid lines) of the cathode electrodes 20 and the cathode electrode post 28 (indicated by broken lines) on the major surface of the silicon pellet of the prior art GTO shown in FIG. 1. As shown in FIG. 2, the cathode electrodes 20 are formed on the n-type emitter layers 18 radially arranged on the p-type base layer 16. FIG. 3 shows a prior art in which island-like patterns of the cathode electrodes 20 are arranged parallel to each other. That is, FIG. 3 is a partial plan view schematically showing a positional relationship between the patterns (solid lines) of the cathode electrodes 20 and the cathode electrode post 28 (broken lines). Note that referring to FIGS. 2 and 3, reference symbol x denotes a region against which the cathode electrode post 28 does not abut.
A crimp stress acting on the cathode electrode of the silicon pellet largely changes near boundary portions of the electrode post 28 indicated by the broken lines in FIGS. 2 and 3. Therefore, if a heat cycle is repeated while such a nonuniform crimp stress is applied, a thermal stress caused by a thermal expansion difference is further applied. Therefore, the silicon pellet having a non-alloy structure may crack within a short time period.
For example, with reference to a schematic partial sectional view of a partially-omitted GTO thyristor having a non-alloy structure and its stress distribution view shown in FIGS. 4A and 4B, respectively, a stress distribution on a cathode electrode surface is as follows. That is, referring to FIG. 4A, an outer diameter of the cathode electrode plate 34 at the cathode electrode side is smaller than that of the cathode electrode pattern 20. In addition, a diameter of a hollow portion in the plate 34 is larger than that of a hollow portion of the pattern 20. That is, although the plate 34 covers the pattern 20, it does not cover the entire surface of the pattern 20. In such a GTO thyristor, stress maximum values are present at the inner and outer boundary ends at which the plate 34 crimps the cathode electrode. Therefore, a major current density at these ends becomes very large.
When heat radiation characteristics of the prior art GTO thyristor shown in FIG. 1 and a GTO thyristor having a non-alloy structure which has the same shape and size as those of the prior art GTO thyristor shown in FIG. 1 and in which an anode electrode plate is not alloy-brazed to a silicon pellet were measured, head radiation characteristics at portions not crimped (regions indicated by reference symbol x in FIGS. 2 and 3) were poor.
When the positional relationship between the cathode electrode and the electrode post is as shown in FIG. 2, currents are significantly concentrated at the crimped end portions in the non-alloy structure. As a result, a withstand voltage with respect to a surge current, a turnoff overcurrent, or the like is reduced.
FIG. 5 is a schematic partial sectional view of a thyristor having a general non-alloy structure. Referring to FIG. 5, a cathode electrode pattern 20 and an anode electrode 24 are partially crimped by cathode and anode electrode plates 34 and 38, respectively. In portions 20a and 24a not crimped by the electrode plates 34 and 38, respectively, currents (indicated by broken arrows in FIG. 5) flowing in a pellet 10 flow through the partial contact portions of the electrode plates 34 and 38, respectively. Therefore, an electric resistance is increased, and a generated heat amount is conducted toward the partial contact portions. Therefore, a thermal resistance in this region is increased to cause insufficient heat radiation.
As described above, a semiconductor device using the silicon pellet having an alloy structure has the above various problems caused by alloy brazing, e.g., a problem in which no uniform crimp pressure can be obtained due to warping of the pellet. In a device using the silicon pellet having a non-alloy structure in which an electrode plate is not brazed in order to solve the above problems, a single silicon pellet is brittle. Therefore, if the silicon pellet is formed into a non-alloy structure by a crimp pressure more uniform than in the alloy structure device, a thermal resistance changes. Therefore, if the shape and size are kept unchanged, no uniform heat radiating effect can be obtained.